Gen. Wissam al-Hassan, a top Lebanese security chief and staunch opponent of the Syrian regime, was reported killed today in a powerful car bomb explosion.

The assassination of Hassan, the head of the Information Branch, the intelligence wing, of the Internal Security Forces, is probably the most significant political killing in Lebanon since 2005, despite his relatively low public profile. It will create significant reverberations in Lebanon, a country torn down the middle over the conflict in neighboring Syria and fearful of Syria’s violence, which has split the country along sectarian lines, spilling over the border. Angry Sunni supporters of Hassan took to the streets in the evening, burning tires at intersections. In the flashpoint city of Tripoli in north Lebanon, gunbattles broke out between factions of Sunnis and Alawites, a Shiite offshoot.

Hassan was politically allied to Saad Hariri, a former Lebanese prime minister who heads the mainly Sunni Future Movement. He played a key role in the arrest in August of Michel Samaha, a former minister and close ally of Bashar al-Assad, the Syrian president. Mr. Samaha was charged with plotting a series of bombings around Lebanon at the behest of the Syrian authorities. The Syrian regime rejected the charges.

The explosive-packed vehicle, which killed at least seven other people, was parked in a narrow street in the Ashrafiyah district, around 100 yards from the bustling Sassine Square intersection, a hub of cafes, banks, and boutique shops. The bomb exploded in mid-afternoon, sending a rumble of thunder across the eastern half of the city. Several cars were destroyed and caught fire, as did at least one adjacent building. Surrounding buildings had balconies blasted off walls and windows in surrounding blocks of flats and offices were shattered, the glass shards crunching under the feet of rescue workers and wide-eyed onlookers.

“It was horrendous. The bomb exploded just as children were walking home from the school. It was very frightening,” says Magda Karam, a housewife who lives nearby.

The car that presumably carried the bomb was mangled beyond immediate identification and apparently hurled some distance down the narrow street by the blast. Voice of Lebanon radio reported that human remains had been discovered at the bomb scene and that the death toll could climb.

“I was shocked by the huge magnitude of destruction, which indicates that the bomb was very large,” said Hatem Madi, Lebanon’s state prosecutor to LBCI television.

The street where the bomb exploded is the site of offices belonging to the March 14 parliamentary coalition which is supported by the West and is critical of the Syrian leadership. The headquarters of the Phalange Party, a Christian organization allied to the March 14 coalition, are also nearby. Before the news broke of Hassan's death, both were thought to be potential targets.

With its complex sectarian rivalries, Lebanon is particularly vulnerable to the possibility of violence in Syria fomenting domestic unrest. The government has attempted to remain aloof from the Syrian conflict, but the country is divided between supporters and opponents of the regime of Syria's President Assad.

Many Lebanese fear a resurgence of the bombings and assassinations of political figures that blighted the country in the three years following the truck bomb assassination of Rafik Hariri, a former Lebanese prime minister and Saad Hariri's father, in February 2005. Prior to his appointment as head of the ISF’s Information Branch, Hassan was in charge of Hariri’s security.

Hassan’s political affiliation with Hariri’s Future Movement earned him the enmity of the Syrian regime and its allies in Lebanon. The arrest in August of Samaha, a sharp veteran politician and once considered untouchable, was a bold step that may have breached a red line. After his arrest, Samaha confessed to the charges of plotting bomb attacks when shown covertly filmed video footage of him discussing the plot with others. The Lebanese judicial authorities also issued arrest warrants for a Syrian general and a colonel for their alleged roles in the plot, which have been ignored by the Syrian authorities.